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It’s now common knowledge that annual changes in the volume of driving no longer follow the old ways.

For sixty years, the number of vehicle miles travelled (VMT) rose steadily almost every year. Predicting more driving miles next year was a foregone conclusion, like predicting that the sun would rise or that computer chips would be faster. The only direction seemed to be up.

Then, after 2004 per-capita VMT turned downward, falling 6 percent, and leading to a decline in total VMT since 2007.

The most recent data are from July, traditionally America’s biggest month for driving. In July 2012, Americans clocked over 258 billion miles behind the wheel, a billion fewer miles than the previous July despite a slightly stronger economy and cheaper gasoline. In fact, you’d need to go back to 2002 to find a July when Americans drove fewer miles than July 2012.

There are good reasons to believe the current slowdown in driving may persist. A report by the U.S. Public Interest Research Group in April showed that youth are leading the trend toward less driving. While the National Household Travel Survey only allows comparison of driving in 2001 and 2009, it shows that Americans aged 16 to 34 reduced their driving miles by 23 percent between those years. Meanwhile, youth are increasing their use of public transit, biking and walking faster than the general population. Changing patterns in the use of information technology and changing preferences for urban living may be major factors in these shifts.

Has America’s long increase in driving turned a corner or just taken a prolonged pause? The answer matters a lot. Consider five scenarios:

If the volume of annual vehicle miles travelled reverts to the average annual increases that took place between 1987 and 2005, then by 2025 the number of vehicle miles travelled (VMT) will grow by more than 27 percent beyond its current 2012 amount of nearly 3 trillion annual miles.

If VMT changes at the average rate it has over the entire period between 1987 and 2012, then it will grow by almost 19 percent by 2025.

If instead VMT changes at the average rate that has prevailed since 2004, then the number of vehicle miles will fall 2.3 percent by 2025.

If VMT changes at the average rate that has prevailed since 2007, then VMT would fall off by almost 8 percent by 2025.

Far more speculatively, if the 23 percent reduction in VMT for drivers under 34 that was observed between the last two National Household Transportation Surveys (2001 and 2009) becomes the trend across all drivers, then VMT would fall by 37 percent over the 13-year period ending in 2025.

The difference spanning these scenarios amounts to almost two trillion vehicle miles per year.

The point is not that we can be certain that driving will decline precipitious, but that we should consider these possibilities when investing in transportation.

The question of whether today’s youth will continue to drive less as they grow older is especially important because the major transportation projects we build today will be used far more by Generation X and Y than by today’s greying transportation leaders. The roads, railways and other infrastructure we build today will be with us long past 2025. The future travel habits of today’s youth should therefore be of paramount importance to transportation policy. Continuing to build new highways at the current pace might arguably make some sense if driving returns to pre-2005 rates of growth. But those outlays would be a colossal waste if more recent trends prevail.

It won’t be surprising if an upturn in the economy leads to some increase in vehicle miles, especially if gas prices don’t also surge. But it’s harder to imagine that we will switch back to the sizeable increases in driving that took place almost every year for six decades after World War II. Even if driving continues to increase at merely the rate of population growth, this would be a major change that should correspond to major changes in transportation investment. While we can’t yet see “the new normal,” it’s a good bet that it won’t be the same as the old normal.

You wouldn’t know that from the last transportation bill, which spends transportation dollars mostly that way the federal government always has.

America should invest more in public transit, biking and walking infrastructure – modes of travel that are expanding, especially among youth. Since we don’t know whether new highways make sense for the future, most road money should go to repairing the existing backlog of deficient roads and bridges. Above all, our elected leaders need to wake up and stop trying to build more of our grandparents’ transportation system. They should take a serious look at the present with an eye to the future.